As is exemplified in the JT9D gas turbine engine manufactured by the Pratt & Whitney Aircraft Group of United Technologies Corporation, the assignee of this patent application and which is incorporated herein by reference, a prediffuser and dump diffuser are disposed downstream of the compressor feeding compressor discharge air to the burner. Typically, compressor air is discharged into the prediffuser which serves to convert a portion of the dynamic pressure head to static pressure whereupon it is then dumped into the dump diffuser for additional conversion, but with high pressure losses. A portion of the now slower moving air is admitted internally of the burner liner to the combustion zones while an additional portion is utilized to cool the liner itself. Because the air is admitted from the periphery and must flow radially toward the center, it is typical to re-accelerate the portion of the air being used in the burner for combustion and dilution purposes with a concomitant pressure drop across the holes in the burner walls. Obviously, there is an associated pressure loss accompanying this type of system.
I have found that I can reduce the pressure losses by discharging the air from the prediffuser into two streams. One stream provides an additional diffuser stage then ducts the air with reduced losses to and around the burner to supply liner cooling air, turbine cooling air and if necessary, small amounts of dilution air to trim the radial temperature profile. The other stream is ducted without the usual dump loss directly into the front end of the burner and supplies the combustion and dilution air. This latter stream without its usual large dump loss and its low loss front feed, has the ability to absorb substantial loss penalties and still provide for a low overall burner pressure loss. This capability is utilized to minimize the pressure losses in the heretofore utilized cooling air circuit.
This invention contemplates that the compressor discharge air leaving the diffuser for feeding the burner shrouds for liner and turbine cooling is captured from the center of the gas path where the total pressure is highest. The ducts that capture this air have expanding cross-sectional areas and commence with zero boundary layer at their walls, and thus function as an additional diffuser stage, further reducing the dynamic pressure head and discharging this cooling air into the burner shroud region with a substantially reduced dump loss. The combination of capturing the highest total pressure from the center of the gas path and the introduction of a second diffuser stage to suppress the dump loss, makes the final total pressure level of the cooling air circuit substantially higher than that of current burner systems. The remaining air not intercepted by the capturing ducts, (the remainder of the high total pressure center region air and all the boundary layer air adjacent to the walls of the prediffuser) is collected by a number of transition sections into discrete pipes where the air is delivered to and into the front end of the burner as combustion and dilution air without the usual diffuser dump loss. Tests that have been conducted have shown that this front-feed combustion and dilution air concept has proven itself to be a low loss burner system.